An aircraft is cleared from "Alfa" to "Kilo" airport by other than filed routing...?

depends.
if failure occurs in vfr conditions, or if vfr conditions encountered after failure, pilot shall continue flight under vfr , land practicable.

if failure occurs in ifr conditions, pilot shall continue flight according following:

(1) route. (i) route assigned in last atc clearance received;

(ii) if being radar vectored, direct route point of radio failure fix, route, or airway specified in vector clearance;

(iii) in absence of assigned route, route atc has advised may expected in further clearance; or

(iv) in absence of assigned route or route atc has advised may expected in further clearance, route filed in flight plan.

(2) altitude. @ highest of following altitudes or flight levels route segment being flown:

(i) altitude or flight level assigned in last atc clearance received;

(ii) minimum altitude (converted, if appropriate, minimum flight level prescribed in §91.121(c)) ifr operations; or

(iii) altitude or flight level atc has advised may expected in further clearance.

(3) leave clearance limit. (i) when clearance limit fix approach begins, commence descent or descent , approach close possible expect-further-clearance time if 1 has been received, or if 1 has not been received, close possible estimated time of arrival calculated filed or amended (with atc) estimated time en route.

(ii) if clearance limit not fix approach begins, leave clearance limit @ expect-further-clearance time if 1 has been received, or if none has been received, upon arrival on clearance limit, , proceed fix approach begins , commence descent or descent , approach close possible estimated time of arrival calculated filed or amended (with atc) estimated time en route.

an aircraft cleared "alfa" "kilo" airport other filed routing. how pilot proceed if radio failure occurs?

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